Linkages which connect attachments such as material or personnel handling devices to a rigid frame, for example, linkages which connect front buckets to backhoe loaders, may be categorized as self-leveling or non self-leveling. Many variant designs of each type exist.
Generally, non self-leveling linkages have tilt mechanisms which are not connected to the vehicle frame, therefore, when raising and lowering the attachment the attachment's attitude remains constant with respect to the linkage and does not self-level with respect to the ground. In order to prevent spillage of material over the back of the attachment as the attachment is raised, it is necessary to tilt the attachment forward in an attempt to maintain its attitude with respect to the ground. This is commonly accomplished by use of a sensor which senses the position of the attachment and activates a hydraulic valve to port hydraulic fluid to a tilt cylinder whose action attempts to maintain the necessary attitude of the attachment with the ground. Such sensors, valves, cylinders, and fluid hosing are often complex, exposed, and vulnerable to damage, these factors combining to create high cost and poor reliability. Further, such systems require continual adjustment as the attachment is raised or lowered which is time consuming, inaccurate, potentially dangerous, and costly in a production environment and a further contributor to poor performance and reliability.
Self-leveling linkages are characterized by having tilt mechanisms which react material loads into the vehicle or equipment mainframe. Tilt linkage geometry is tailored such that as the attachment is raised, the attachment's attitude toward the ground automatically adjusts to produce a predetermined leveling characteristic. Self-leveling linkages are typically complex in terms of number of parts and are costly to produce and assemble. However, typically, no adjustment is required by the operator to maintain the attachment's attitude toward the ground as the device is raised or lowered.
When using certain attachments, such as a front loader bucket, for ease of operation it is desirable to have an automatic means that will return the bucket after dumping to a position that as the bucket is lowered it is level with respect to the ground. Mechanisms employed with non-self leveling linkages to prevent spillage as the bucket is raised typically override such automatic means and prevent the bucket from rotating far enough back to be level with the ground when the bucket is lowered. Self leveling linkages typically do not exhibit this problem.
When the attachment is a pallet forks, parallel lift is desired. Non-self leveling linkages require operator input to maintain level and rely upon hydraulic system pressure to perform required tilt corrections. For obvious reasons, non self-leveling linkages are generally poor in terms of pallet fork performance. Self-leveling linkages are far superior for pallet fork leveling in that the linkage may be tailored for parallel lift without need for operator input tilt corrections.
It is the object of the present invention to provide automatic self-leveling linkages which use fewer components than known prior art which thereby translates into reduced manufacturing costs and improved reliability.
It is a further object of the present invention to provide a linkage which will make automatic adjustments to the attachment to provide exact parallel lift for any single predetermined attitude of the attachment.
It is a further object of the present invention to provide a linkage which may be tailored in the factory or at the job site for a variety of alternative paths of motion.